I have a female, Gemma, that I suspect to be a lethal yellow banded, though I'm in no way an expert. In my little research I saw that mice with a lethal yellow are prone to obesity. My little girl isn't quite so little, she's about 44g while her littermate is only 33g. And it's not just that her sister is smaller, Gemma is FAT. She has a big rounded tummy that's quite noticeable. (And there's absolutely no chance of her being pregnant, no worries there.)

I don't want this to harm her health but I'm not sure how to help. She's housed with four other females that share one wheel. They all get a turn typically, though I want to get a second wheel for them. The two fattest, Gemma the lethal yellow and Layla who has always been my biggest mouse, are the ones I see on it hardly ever. Layla isn't so much fat as she is large, though a little pudgy. Gemma is fat though, 100%.

Food wise, they're fed a mix (Kaytee Forti-Diet Pro for mice/rats/hamster, Kaytee Complete for hamster/gerbil, uncooked pasta, oats). I want to make a perfect mix for them but I haven't been able to edit my mix for quite awhile (school, work, and home life have gotten in the way). But their food is also part of the problem, I'm sure.

Suggestions for helping some mice lose weight? (Also, any information on lethal yellows and obesity would be appreciated!)

Lethal yellow are very rare in the US, they are pretty much exclusively found overseas. And although any mouse can be obese, Brindles are the ones that often carry the gene. Those mice can get upwards on 80g! So i wouldn't worry about 44g. In fact many of my mice are about 40-50g. Also, even if your mouse died become obese, we don't recommend restricting their food intake because they end up suffering nutritionally. And that is more harmful that being overweight.

i would suggest switching their food as well. Many of us use vitakraft vitasmart parakeet food as a base mix and add to that, although is fine how's it is, nutritionally speaking. But since mice appreciate variety, i add things like flaxseeds, quinoa, oats, nuts, pumpkin seeds, dried mealworms, cheerios, and coconut.

Ditto what Scaredy said. I just wanted to add that you can try to up their (supervised) out of tank playtime. Just make sure they don't go without access to water for longer than 30 minutes. Either offer a water bottle in the play area or you can do a few 30 minute play sessions a day. You can also slowly up their fresh veggies a little more, just make sure it's not overtaking their main food (seed mix). A minimum of twice a week fresh foods is recommended but small amounts can be offered every day. Go slowly to prevent stomach upset and watch for allergies.

I agree that 44 grams doesn't sound like a lot, unless she is built very tiny. A picture helps determine weight better than a number on a scale though.